In the art of magnetic resonance imaging, it is known to employ four-dimensional (4D) respiratory phase-guided imaging methods for accurate characterization of motion of tumors and/or organs due to respiration of a subject of interest, usually a patient, particularly for abdominal tumor motion tracking.
Further, T2-weighted magnetic resonance imaging is known to provide better tumor-tissue contrast than T1- or T2/T1-weighting but requires long pulse sequence repetition times TR in the range of about 3 s. For using long pulse sequence repetition times TR, different slices of the tumor and/or the organ have to be measured over several breathing cycles.
In the paper by Y. Hu et al., “Respiratory Amplitude Guided 4-Dimensional Magnetic Resonance Imaging”, Int J Radiation Oncol Biol Phys, 86 (1), 198-204 (2013), a method of acquiring 4D respiratory phase-guided magnetic resonance images is described which comprises a scheme of interleaving the acquisition of different points in time for different slices to be imaged in order to reduce a total acquisition time.
The acquisition scheme is exemplarily illustrated in FIG. 5 for a scan package comprising M=8 slices and N=4 respiratory phases (0%, 50% inspiration, 100%, 50% expiration). The acquisition scheme can be thought of as two nested loops, wherein an inner loop varies a slice index m (m=1, . . . , M), and an outer loop having an index n is executed N times, controlling the respiratory phases. As can be obtained from FIG. 5, not all slices in the inner loop are acquired for the same respiratory phase. Instead, the respiratory phase is selected according torespiratory phase=((m−1)+(n−1))% N wherein “%” denotes the modulus operation. The first scan sequence includes magnetic resonance images of the eight slices at respiratory phases n=0, 1, 2, 3, 0, 1, 2, and 3. The second scan sequence includes magnetic resonance images of the eight slices at respiratory phases n=1, 2, 3, 0, 1, 2, 3, and 0. The third scan sequence includes magnetic resonance images of the eight slices at respiratory phases n=2, 3, 0, 1, 2, 3, 0, and 1. The fourth scan sequence includes magnetic resonance images of the eight slices at respiratory phases n=3, 0, 1, 2, 3, 0, 1 and 2.
As is described by Y. Hu et al., using triggers at preselected respiratory levels enables acquiring MRI images at different respiratory states in different respiratory cycles and, thus, eliminates the restriction on the long pulse sequence repetition time TR. By that, more magnetic resonance imaging sequences, in particular T2-weighted magnetic resonance images, are compatible with 4D magnetic resonance imaging.